Possibly all exchanges that are presently built have this basic construction. Such an exchange is described in "SYSTEM 12--Preview of the Fundamental Concept", R. Van Malderen, Electrical Communications, volume 59, no. 1/2, 1985, p. 20-28.
The primary task of the switching network consists in establishing connections between any two subscriber terminals. Each of these subscriber terminals may be connected to a peripheral unit of this exchange, or to another exchange, which is then connected to another peripheral unit.
For practical reasons, all transitions between a peripheral unit and the digital switching network are performed in the same way. The capacity of such a transition is coupled to the capacity of a conventional multiplex connection. In the above mentioned example of a switching network according to SYSTEM 12, 60 connections of 64 kbit/s each can be transmitted in both directions between a peripheral unit and the digital switching network. To this are added two times two special channels of 64 kbit/s each as well.
Additional transmission capacity is required in a switching network, beyond the above described incoming and outgoing traffic, for the connection of subscriber terminals to each other. It serves the most varied purposes, e.g. to signal when a connection is established or terminated, the connection of the controls of the peripheral units to each other or to a central control unit, the distribution of clock rate or tones, or the updating of data or control programs.
The switching network is also used in part for these additional purposes, in part the exchange contains separate networks for this purpose. In the above mentioned SYSTEM 12 example, a distribution network for clock rate and tones is provided; incidentally, only the digital switching network is available for internal communication (see FIG. 3 on page 21 of the above mentioned article).
Furthermore, in all exchanges built in this way, the increasingly required possibility of establishing N-channel connections can only be achieved at great expense, because internal blockage takes place very quickly when the switching network has to establish several synchronous connections.